A nitrogen oxide (NOx) sensor is known as a sensor that detects the concentration of NOx as one of the specific gas components contained in the exhaust gas discharged from an internal combustion engine. The NOx sensor is placed on, for example, the downstream side of a selective reduction-type NOx catalyst in a diesel engine exhaust gas purification system using the NOx catalyst. The NOx concentration that is detected by the NOx sensor placed as described above is used for the control of a reducing agent addition amount added to the NOx catalyst.
Also known is a NOx sensor that has a first pump electrode in a first chamber and a second pump electrode and a pump electrode for NOx detection in a second chamber (refer to, for example, Patent Literature 1). According to Patent Literature 1, the first pump electrode causes a decomposition reaction by means of inter-electrode voltage application and discharges oxygen out of the first chamber. The second pump electrode discharges oxygen out of the second chamber by means of inter-electrode voltage application. The pump electrode for NOx detection reacts to NOx gas in gas by means of inter-electrode voltage application and outputs a NOx concentration in the form of a current signal.
Each cell that constitutes the NOx sensor deteriorates as a poisoning substance is deposited on its cell surface as a result of a catalytic reaction and this deterioration results in a decline in the detection accuracy or detection sensitivity of the NOx sensor. Examples of the poisoning substance include those derived from manganese and silicon, which are additives contained in fuel oil.
According to the NOx sensor deterioration determination method that is disclosed in Patent Literature 1 below, the NOx concentration in exhaust gas reaching a NOx sensor is forcibly fluctuated and an abnormality is determined in a case where the fluctuation of an output value outputted by the NOx sensor at that time deviates from a normal fluctuation.